APOE and Alzheimer disease: However, the APOE4 status may not be the only susceptibility factor and it would be interesting to identify other factors: (i) genetic factors as, beyond APOE4, other genetic risk factors of AD are also linked to HSV [19, 20] or involved in the immune response; (ii) factors linked to the virus such as the type of viral strain; or (iii) factors linked to the immune capacities of the host.